Summary This Program encompasses three tightly connected themes. The primary theme is centered on protein phosphatase 2A (PP2A), an established tumor suppressor. Understanding how PP2A serves as a tumor suppressor has been challenging for several reasons. First, PP2A is a heterotrimeric enzyme composed of A, B and C subunits, each with several members giving rise to > 80 possible PP2A holoenzymes. Genome characterization studies show that many of the PP2A subunits are recurrently mutated or deleted in human cancers in a manner that disrupts PP2A assembly or catalytic activity. Thus, PP2A is a highly complex and diverse enzyme family, making deconvolution of its tumor suppressing functions problematic. This Program has developed a global phosphopeptide analysis system that has begun to identify new PP2A substrates and promises to accelerate progress in deconvoluting the nature of its complex and diverse tumor suppression roles. The second theme of this Program is the use of DNA tumor virus oncoproteins as smart probes to understand PP2A tumor suppressor function. All members of this program have shown that PP2A is a critical player in cell growth regulation and transformation and that viral oncoproteins interact with PP2A in ways that interfere with PP2A tumor suppressing activity and contribute to transformation. The third theme begins with the knowledge that ~ 20% of global cancer is caused by infectious agents. Merkel cell polyomavirus (MCPyV) is the clear cause of most Merkel cell carcinomas (MCC), a highly lethal skin cancer. MCPyV encodes a small T antigen (ST) that is essential for MCC and can transform human cells on its own. MCPyV ST interacts with PP2A and this Program will investigate the functional output of Merkel ST - PP2A interactions and search for novel insights into the mechanisms by which MCPyV contributes to MCC. The goals of this Program are: (1) To identify PP2A-driven signaling cascades that contribute to clinically important examples of tumor suppression; (2) To identify cancer-relevant PP2A substrates. (3) To seek ways to restore or activate mutant PP2A function. Each of the four Projects and the Functional Proteomics Core will focus on how specific PP2A perturbation, whether by mutation or association with oncoproteins, targets pathways that control cancer development. Project 1 will study the affects of MCPyV ST on MTOR signaling and DNA break repair. Project 2 will assess the effects of murine polyomavirus interactions with PP2A upon YAP - Hippo signaling. Project 3 will study the tumor suppressor activity of the PP2A-A? subunit that influences AKT and SRC signaling pathways. Project 4 will study the newly detected role of specific perturbations of the PP2A-STRIPAK complex in the molecular pathogenesis of cancer. The Functional Proteomics Core will identify the subunit composition of relevant PP2A complexes and their phospho-protein substrates. This Program will bring important, new insights into the largely uncharacterized cancer suppressor role of PP2A. We expect to identify important signaling pathways regulated by PP2A to suppress cancer development.